Water Sources, Quality, and Characteristics
Source-water characteristics drive every downstream decision, so questions often test whether you can match a water type to its likely problem. Groundwater from a confined aquifer is typically low in turbidity and microbial load, cool, consistent, and often high in hardness, iron, manganese, arsenic, radon, or sulfide; it may be anoxic and corrosive. Surface water is variable, warmer, higher in turbidity, color, natural organic matter, and pathogens, and responds quickly to storms and seasons. Unconfined aquifers sit beneath a water table and are more vulnerable to surface contamination than confined aquifers.
Know the meaning of the core characteristics: turbidity (light scattering by suspended particles, NTU), true versus apparent color (apparent includes turbidity), alkalinity (acid-neutralizing capacity, mostly bicarbonate), hardness (polyvalent cations reported as CaCO3), pH, temperature, conductivity/TDS, and dissolved gases. Alkalinity and hardness are constantly confused: alkalinity is a capacity measure, hardness is a concentration of calcium and magnesium.
Question traps: assuming low turbidity means microbiologically safe (it does not); assuming groundwater needs no disinfection; treating pH and alkalinity as the same thing; forgetting hardness is expressed as CaCO3; and ignoring that iron and manganese cause staining and taste complaints but are secondary, not health, issues.
Worked example - hardness as CaCO3. A sample has 40 mg/L Ca and 12 mg/L Mg. Equivalent factors: 50/20 = 2.5 for calcium; 50/12.15 = 4.12 for magnesium. Hardness = (40 x 2.5) + (12 x 4.12) = 100 + 49 = 149 mg/L as CaCO3 - moderately hard.